Itertools instead of nested loops

Question

list_a = [] 
for color in [True,False]:
    for piece in range(1,7):
        list_a = list_a + function(piece,color)

Here the function(piece,color) returns a list, which I want to join and finally return the long list, can itertools.chain be used here? because I think it might be faster. I am only displaying an example, but in my actual code the loop runs about 100,000 times, which is why I am looking for a faster method.

Answer 1

I'm going to answer the question you should have asked instead ;-)

This:

list_a = list_a + function(piece,color)

takes time quadratic in the number of times it's executed. Each time, a brand new list object is created, copying the entirety of the old list_a and the new list.

So if it's executed many times, you can get a huge improvement by changing it to this:

list_a.extend(function(piece,color))

Then list_a is extended "in place" whenever possible; under the covers, it may need to make a copy to a larger memory area from time to time, but overall the amortized time is linear in the number of times it's executed.